Sometimes the global "Open With" setting in "Get Info" works more reliably if you first apply it to just the selected document, close the Get Info window, then re-open Get Info and then click the button to "Change All".

If I Get Info on a .nwm file and set to open with Nisus and then click Change All (whether I've closed the Get Info and reopened or not), the setting for the file in question *reverts to TextEdit*. wtf?

I take it that this is an OS problem so I won't hold my breath for a fix.

Perhaps you can fix it by using a free preferences pane Default Apps.http://www.rubicode.com/Software/RCDefaultApp/
Install it, open it, switch to Extensions tab and choose Nisus Writer Pro as the Default Application for nwm. If nwm is not listed, add it.

Also, try to rebuild Launch Services database using a utility such as Onyx.